A couple of weeks ago, I noticed my driver's side wiper arm was not parking in its usual position. I can tell if it's sitting high or low in relation to the space between the bottom of the windshield and the end of the hood. It had been sitting too high, so I loosened the retaining nut, reseated the arm and retightened the nut and with a lock washer. Today, I noticed it the same way, but this time, the nut was nice and tight. I repositioned the arm and again retightened the nut and while spraying fluid on the windshield, and stopped the wiper cycle, it stopped right in my line of sight on the windshield. The nut was tight. Question. Is there something behind the cowl where the mechanical guts are that would cause this to drift around like this? I recall once when pulling the cluster out to change bulbs, I saw the cycle arm that comes across from the wiper motor.
I don't want this to be a problem while driving in a downpour.

Thanks.
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'86 sedan with 342K miles with original engine and automatic transmission